This invention relates to a digital click removal and squelch control circuit for an FM receiver which, preferably is completely digital in operation. More particularly, the invention relates to a click removal and squelch control circuit which operates on a series of binary number electrical signals, occurring at equally-spaced time intervals, each of which is representative of the instantaneous phase angle of a received frequency-modulated electrical signal or signal derived therefrom. The click removal circuit differentiates and, thus, demodulates the binary number electrical signals representative of phase angle, and removes or discards those of the differentiated binary number electrical signals which represent electrical noise or clicks. Also, if a predetermined number of clicks occur during a predetermined time interval, squelch control circuit means provides a squelch control signal to disable the output of the FM receiver.
The most widely used method of implementing squelch control in analog FM receivers consists of bandpass filtering of out-of-band noise at the output of the receiver's discriminator and disabling the audio amplifier whenever the average value of the rectified noise exceeds a preset threshold. The operations of bandpass filtering, rectification, lowpass filtering, and threshold decision can be implemented digitally in FM digital receivers. However, individual digital operations to replace the analog counterparts thereof would in general result in inefficient implementation.
Illustrative of prior art digital approaches to squelch control or electrical noise control are U.S. Pat. Nos. 3,437,937 to Warfield, 3,678,396 to Hoffman and 3,633,112 to Anderson.
In a digital FM receiver, a received frequency-modulated signal is sampled at equally-spaced time intervals to produce binary number electrical signals representative of the sampled values of the received signal. The binary number signals thus produced are down-converted or translated in frequency, utilizing digital techniques, to produce a series of binary number electrical signals having the carrier frequency removed. These translated binary number electrical signals are supplied to an arctangent demodulator which produces at its output binary number electrical signals representative of the instantaneouos phase angle of the received signal or a signal derived therefrom. The time derivative of the binary number signals representative of phase angle produces binary number electrical signal output samples representative of the instantaneous frequency of the received signal. Electrical noise or clicks appear as overly large changes in the phase angle between successive samples representative of the instantaneous phase angle.
In the digital click removal and squelch control circuit hereinafter described, the sampling rate at the output of the arctangent demodulator should be higher than the highest frequency component of the modulating signal. Preferably, in order to retain all of the information present at the input of the arctangent demodulator, the sampling rate should not be lower than the FM bandwidth, and preferably is on the order of one and one-half times the FM bandwidth.